1. Field of the Invention
The present invention relates generally to a composition for ceramic substrates, especially those having desirable dielectric and mechanical properties for application in high frequency ceramic devices. More specifically the present invention provides glass ceramic substrates for the manufacture of ceramic circuit components and modules as well as any other ceramic devices for microwave applications.
2. Description of Related Art
The increased volume of communication information transmitted in recent years has promoted the rapid development of various communication systems utilizing microwave bandwidth regions, such as cellular telephones, satellite communication and satellite broadcasting systems. Accompanying these communication system advances has been the development of various microwave dielectric materials to accommodate the industry's needs. Such advances in wireless applications are highly dependent upon improvements in microwave materials and new developments in processing methods for microwave components and devices.
In general, microwave applications require dielectric materials which have low dielectric constants and low dielectric loss (indicated by a low dielectric loss factor) in order to increase the Q factor (the inverse of the dielectric loss factor) and minimize energy absorption by the dielectric material that would reduce resonant signal intensity. Accordingly, a relatively lower loss factor results in a relatively higher Q factor. Additionally materials with a lower dielectric constant, and lower dielectric loss factor (high Q factor), are essential in order to maintain the physical dimensions of the electronic devices in a range conducive to manufacturing limitations. Glass ceramic is one of the best candidates as a substrate material for microwave electronic components and devices because of its excellent comprehensive properties.
The dielectric properties of glass ceramic are determined not only by its composition, but also by its sintered microstructure. Glass ceramic is usually produced by ceramic fillers that are mixed with glasses and then fired at a high temperature (Examples of glass ceramics include the borosilicate glasses with alumina system developed by Fujisu and the lead borosilicate glass with alumina system developed by Dupont). A fully densified glass ceramic microstructure is conducive to achieving a high Q factor and mechanical strength. Moreover, the reduced interactive layer thickness requires a fully dense interlayer of insulator (glass ceramic) to avoid shorting any conductive circuits.
Many efforts have been made in order to develop glass ceramic dielectric materials for microwave application (see U.S. Pat. No. 6,403,199, U.S. Pat. No. 3,926,648, U.S. Pat. No. 6,121,173, U.S. Pat. No. 4,642,148, U.S. Pat. No. 6,080,693, U.S. Pat. No. 4,879,261, U.S. Pat. No. 6,228,788, U.S. Pat. No. 4,755,490, and U.S. Pat. No. 5,747,396).
For example, U.S. Pat. No. 6,228,788 describes a ceramic composition for producing high-frequency ceramic inductors, which can be densified up to 95% at temperatures between 800-1000° C. The composition consists of: (a) 20-80% by weight of a borosilicate glass comprising 10-14% by weight of B2O3, 90-80% by weight of SiO2, 0.1-4% by weight of Al2O3 and 0.1-4% by weight of alkali metal oxides; and (b) 80-20% by weight of a filler of Al2O3. While this glass ceramic can be well densified, the dielectric constant of such material is about 6, which is too high for microwave applications. In general, the dielectric constant is not low enough for microwave applications in substrates in which Al2O3 is used as filler.
In another example, U.S. Pat. No. 5,747,396 discloses a glass ceramic composition using a single borosilicate glass having a low softening point and silica as a filler. While the invention of this patent addressed a manufacturing problem that originated from borosilicate glass because of its unstable composition of B2O3, and reduces the dielectric constant of the resulting substrate to about 5.3, the material is not able to be adequately densified. For glass ceramic, which is a mixture of glass and ceramic powders, the sintering kinetics are controlled by the viscous flow of low softening point glass, because more than half the volume percent of glass is present in the glass ceramic mixture. By using a glass with low softening point, the viscous flow of the molten glassy phase will form a closed glassy phase network very quickly. Consequently, the gases will be trapped within the glass network and leave a porous matrix in the intermediate and final densification stages. As such, the glass ceramic substrate is not fully densified.
Finally, in recent years the electronic industry as a whole has been moving to lead-free assembly processes for environmental and market concerns. Though lead is one of the oldest known materials used by man, recent studies have shown that lead which accumulates in the body can lead to brain, liver and kidney damage. Although the electronics industry accounts for less than 1% of the annual lead consumption, the main drive for its elimination is the risk that lead can seep from land-fill dumps of electronic waste to the ground water.
It is therefore desirable to provide an improved composition for forming fully densified lead-free ceramic substrates having desirable dielectric properties which overcomes the drawbacks in the prior art.